It's Study Sunday! According to research by Dr. Peter Borshelt, it was found that by the time puppies reached 5 months of age, a good 50 percent of them were already lifting their legs, and then, by the age of 8 months, 50 percent of them were urine marking. Then, by the age of 24 months, the great majority of dogs (90-95 percent) were found to be respectively urine marking and leg lifting.